NUMERICAL SIMULATIONS OF AN OROGRAPHICALLY-INDUCED MESOSCALE CLOUD SYSTEM AND SUBSEQUENT CYCLOGENESIS OFF THE SOUTH COAST OF JAPAN

In cold seasons, a low-level mesoscale cloud system associated with an orographically-induced shear line often develops off the south coast of central Japan. Numerical experiments using high resolution limited- area models based on Japan Spectral Model (JSM) were performed for the 14 October 1991 case, where active development of the cloud system an d associated cyclogenesis occurred. The model reproduced the evolution of the disturbance well, which is described as follows. In the initia l stage of the cloud formation, a strong low-level northerly wind in t he upwind side exhibited a splitting pattern around the Chubu Mountain s, while the wind in the lee side was weak. High theta e air that was formerly brought by a typhoon was trapped in the south coast area. The n an easterly wind gradually intensified along the south coast of Kant o, which caused the westward migration of the cloud system, and a NW-S E oriented shear line with strong positive vorticity in the lee of the Chubu Mts. A band-shaped precipitation area was simulated along the n ortheast side of the shear line, whereas on the southwest side, there was a dry air mass that descended the south slope. At the 18-h forecas t, a shallow, mesoscale cyclone appeared on the shear line, which was nearly in geostrophic balance with the strong cyclonic circulation. Th e mesoscale cyclone then combined with the middle-level trough that pr ogressed along the south coast and developed further as a synoptic-sca le, extratropical cyclone. Simulated results show that the initial gro wth of the disturbance is triggered by the orographic effects, and tha t the later development is strongly influenced by the approach of the middle-level trough.